Intracellular potentials and the intracellular activities of Na2 and K2 were examined using conventional and ion selective microelectrodes. There was a positive correlation between short-circuit current and intracellular Na ion activity. The data could be fit by a Michaelis-Menten relationship. Treatment of the tissue with ouabain and amiloride produced an increase and a decrease, respectively, in the intracellular Na ion activity. Treatment with aldosterone produced a large increase in short circuit current with a substantial increase in intracellular Na ion activity. The increase in intracellular Na ion activity implied increased apical permeability to Na ion. The increased short-circuit current can be completely explained only if additional transport sites are recruited. Intracellular Na ion activity does not seem to directly affect apical membrane permeability.